Can I put just the EJ257 block with crank, pistons, and rods, onto my current 03 wrx. Will everything bolt right up or do I need anything else? If so what else do I need to do? Also will it affect the transmission?

yes everything is a direct fit. You will need tuning for it and might need a thicker head gasket unless you want to run a octane booster like Torco(no 93 where I live). Bigger than stock turbo would be advisable as well. Transmission will last as long as you dont beat on it like any other mod. I have been running my hybrid for almost a year on the stock tranny.

well, darthsofearu wanted to know and he at least did some searching. Got no problem with dragging up old threads if there's a good reason.

TY for info Jave. darthsofearu, the key is using the WRX heads and creating a "hybrid" rather than swapping the whole motor in there. That way you still keep the stock ECU etc and it's easy to just re-tune like you would for most other power mods.